Carotid endarterectomy prevents stroke and reduces mortality. Despite its low incidence of major medical complications, patients having carotid endarterectomy commonly manifest significant post-surgical cognitive deterioration that is mental deterioration, in ~ 25%. The procedures used during surgery may be critical in reducing or enhancing the likelihood of this post-surgical mental deterioration. Increasingly, carotid artery stenting is a procedure that may replace carotid endarterectomy, but it also has been associated with post-surgical persistent cognitive deterioration in a substantial proportion of patients. We aim to determine whether cognitive function is improved by increasing cerebral blood flow during carotid artery surgery via inserting a shunt. We will randomize patients to either conventional management, which includes increasing systolic blood pressure, or by inserting a shunt while maintaining systolic blood pressure at baseline values. All patients will be examined before and after surgery (1 day, and 1 and 6 months [1 d/1 & 6 m]) with a battery of neuropsychometric tests. This study will test the hypotheses that use of a shunt reduces the rate, extent and persistence of cognitive impairment following carotid endarterectomy with an associated effect on two serum markers of injury to the brain, S100B, a protein from glia, and NSE, a protein from neurons, and that a marker sensitive to blood flow during surgery, namely how much oxygen is extracted by the brain, will show reliable associations with cognitive outcomes and the two biomarkers of tissue injury, S100B and NSE. Also, we aim to determine the incidence of cognitive deterioration associated with carotid artery stenting by comparing cognitive performance in patients having carotid artery stenting to those having coronary artery stenting using a battery of neuropsychometric tests before and after surgery (1 d/ 1 & 6 m). This study will test the hypotheses that the use of stenting does not reduce the rate, extent and persistence of cognitive impairment compared to carotid endarterectomy, and will be associated with elevations in S100B, MRI evidence of focal infarct, and quantitative transcranial Doppler evidence of emboli. Based on the findings from these studies we will reduce cognitive injury, stroke and mortality from these procedures. PUBLIC HEALTH RELEVANCE Carotid artery stenosis is a major cause of stroke and mortality. Its treatment is to remove the stenosis either surgically by performing a carotid endarterectomy or endovascularly by performing carotid artery angioplasty and stenting (stenting). While we have demonstrated that carotid endarterectomy leads to cognitive dysfunction in ~25% of patients, more so in patients with Type 2 diabetes mellitus, no direct comparison of the incidence of cognitive dysfunction has been performed between patients undergoing carotid endarterectomy and carotid artery stenting. The aims of this study are to perform just such a comparison to see if one treatment is safer than the other, and also to determine whether cerebral blood Flow-Enhancement with shunt placement across the surgical site during carotid endarterectomy in patients with diabetes mellitus reduces cognitive dysfunction.